Simulation of conservation practice effects on water quality under current and future climate scenarios

Analysis of the effects of implementing different conservation practices, as well as increased levels of conservation practices under existing and projected future climate, will determine if current conservation practice recommendations will be sufficient to maintain soil and water resources. The Soil and Water Assessment Tool (SWAT) was used to study four watersheds of different sizes (CCW = 680 km2, F34 = 183 km2, AXL = 42 km 2 and ALG = 20 km2) located in Northeastern Indiana. The overarching goal of this study was to evaluate the effect of various agricultural practices on runoff and agricultural chemical losses under current and future climate conditions, using an appropriately calibrated SWAT model. The results indicated calibrating SWAT at one watershed size and applying the optimized parameters to watersheds of different sizes with similar physiographic features produced satisfactory predictions of streamflow, nitrogen and phosphorus losses. Between the baseline period (1961-90) and the end of this century (2099), average annual precipitation for the watershed is expected to increase by approximately 8.5%, average daily solar radiation will increase by approximately 2.4% and average annual maximum and minimum temperatures will increase by approximately 3.9 and 4.0° C respectively. Based on SWAT simulations, changes in future climate resulted in decreased surface runoff (9% to 22%) and increased tile flow (20% to 25%) because more precipitation occurred in smaller events, allowing more infiltration to occur. There was an increase in sediment loss for all four watersheds (ranging from 6% to 30%), while average annual soluble-P loss decreased for the CCW (10%) and F34 (25%) watersheds between the baseline period and the end of this century. Changes in atrazine, soluble-N, total-N and total-P losses were not significant at α = 0.05 Given the changes in projected future climate, the long-term impacts of both individual and combined conservation practices were assessed in the AXL watershed. The estimated average annual reductions for each decade of future climate due to conservation practices implementation ranged from 15% to 25% for surface runoff, 32% to 68% for sediment, 37% to 60% for atrazine, 5% to 13% for soluble-N, 12% to 35% for total-N, 9% to 41% for soluble-P, and 33% to 60% for total-P. Results of the study indicated that individual conservation practices were effective in reducing a targeted pollutant load, but combined practices were more effective in reducing multiple pollutant loadings simultaneously. No-till was the most effective individual conservation practice, while a combination of five conservation practices was most effective in reducing runoff, sediment, atrazine, and nitrogen and phosphorus losses

Evaluating the Effects of Watershed Size on SWAT Calibration

The Soil and Water Assessment Tool (SWAT) has been calibrated in many watersheds of various sizes and physiographic features. However, it is still unclear whether SWAT calibration parameters will produce satisfactory results if they are implemented in watersheds of different sizes. Evaluating the transferability of SWAT calibration parameters between watersheds of different sizes will provide insight into whether it is acceptable to calibrate SWAT in one watershed and apply the optimized parameters in different size watersheds by assuming both watersheds have similar physiographic properties. This study investigated the influence of watershed size on the SWAT model calibration parameters transferability between four watersheds (CCW = 680 km2, F34 = 183 km2, AXL = 42 km2, and ALG = 20 km2) located in Northeastern Indiana. The results show that calibrating SWAT at one size and applying the optimized parameters at different watershed sizes of similar physiographic features provided satisfactory simulation results. The size watershed at which SWAT was calibrated had little effect on streamflow predictions. Soluble nitrogen loss estimates were improved when calibration was performed at the larger CCW watershed while calibrating SWAT at the smaller AXL and ALG watersheds produced improved statistical indicator values (NSE, R2, and PBIAS) for soluble P and total P when applied to the larger CCW and F34 watersheds

Impacts of Global Circulation Model (GCM) bias and WXGEN on Modeling Hydrologic Variables

A WXGEN weather generator is commonly used to generate daily climate data for Soil and Water Assessment Tool (SWAT) model when input climate data are not fully available. Of all input data for WXGEN, precipitation is critical due to its sensitivity to the number of wet days. Since global climate model (GCM) data tend to have excessive wet days, use of GCM precipitation data for WXGEN may cause errors in the estimation of climate variables and therefore SWAT predictions. To examine such impacts of GCM data, we prepared two climate data for SWAT using WXGEN with both the original GCM data with the excessive number of wet days (EGCM) and the processed GCM data with the reasonable number of wet days (RGCM). We then compared SWAT simulations from EGCM and RGCM. Results show that because of the excessive wet days in EGCM, solar radiation generated by WXGEN was underestimated, subsequently leading to 143 mm lower ET and 0.6–0.8 m3/s greater streamflow compared to the simulations from RGCM. Simulated crop biomass under EGCM was smaller than RGCM due to less solar radiation. Although use of WXGEN is increasing in projecting climate change impacts using SWAT, potential errors from the combination of WXGEN and GCM have not well investigated. Our findings clearly demonstrate that GCM bias (excessive wet days) leads WXGEN to generate inaccurate climate data, resulting in unreasonable SWAT predictions. Thus, GCM data should be carefully processed to use them for WXGEN

Evaluating Concentrated Flowpaths in Riparian Forest Buffer Contributing Areas Using LiDAR Imagery and Topographic Metrics

Riparian forest (CP22) buffers are implemented in the Chesapeake Bay Watershed to trap pollutants in surface runoff thus minimizing the amount of pollutants entering the stream network. For these buffers to function effectively, overland flow must enter the riparian zones as dispersed sheet flow to facilitate slowing, filtering, and infiltrating of surface runoff. The occurrence of concentrated flowpaths, however, is prevalent across the watershed. Concentrated flowpaths limit buffer filtration capacity by channeling overland flow through or around buffers. In this study, two topographic metrics (topographic openness and flow accumulation) were used to evaluate the occurrence of concentrated flowpaths and to derive effective CP22 contributing areas in four Long-Term Agroecosystem Research (LTAR) watersheds within the Chesapeake Bay Watershed. The study watersheds include the Tuckahoe Creek watershed (TCW) located in Maryland, and the Spring Creek (SCW), Conewago Creek (CCW) and Mahantango Creek (MCW) watersheds located in Pennsylvania. Topographic openness identified detailed topographic variation and critical source areas in the lower relief areas while flow accumulation was better at identifying concentrated flowpaths in higher relief areas. Results also indicated that concentrated flowpaths are prevalent across all four watersheds, reducing CP22 effective contributing areas by 78% in the TCW, 54% in the SCW, 38% in the CCW and 22% in the MCW. Thus, to improve surface water quality within the Chesapeake Bay Watershed, the implementation of riparian forest buffers should be done in such a way as to mitigate the effects of concentrated flowpaths that continue to short-circuit these buffers